The present invention relates to a method for measuring an amount of cholesterol in low density lipoproteins (LDL), which has been used for important factor for diagnosis of arteriosclerosis, disorder of lipid metabolism, etc. in the field of clinical chemistry test and also to a reagent used in this method.
Lipids in serum exist as lipoproteins and they are classified according to their density into chylomicrons, very low density lipoproteins (VLDL), LDL and high density lipoproteins (HDL). They are particles comprising proteins covalently bound to lipids and have a micelle-like spherical structure composed of a non-polar core consisting of triacylglycerol and cholesterol ester on one hand and proteins, cholesterol and phospholipid covering the core on the other hand, and apoproteins and lipids are different according to each lipoprotein. LDL has a particle size of about 180 to 280 angstroms in diameter and specific gravity of a range of 1.006 to 1.063 (g/ml) and its apoprotein is composed mainly of apoB-100, and 80% thereof is occupied by lipid which contains a large amount of cholesterol. On the other hand, HDL is the smallest lipoprotein having a particle size of about 50 to 120 angstroms in a diameter and such higher specific gravity as 1.063 to 1.125 (g/ml), which is enriched with proteins (mainly apoA, about 50% thereof) containing lipid composed of mainly phospholipid and cholesterol. LDL has a role of transporting cholesterol synthesized in liver to peripheral cells and HDL has, on the other hand, a role of transporting it from peripheral tissues to liver.
Among those lipoproteins, LDL has an action causing atherosclerosis which is most frequently found among arteriosclerosis (progressive disease caused by sedimentation of lipids in cells of smooth muscles in inner walls of arteries, particularly of coronary arteries), and thus an amount of cholesterol originated therefrom (LDL-cholesterol) has been remarked as a risk factor of arteriosclerosis.
However, there has been found no method for measuring LDL-cholesterol accurately after a simple procedure.
For instance, LDL-cholesterol amount obtained by the following equation proposed by Friedewald, etc. has been said as containing about 10% error; EQU LDL-cholesterol amount=total cholesterol amount-HDL-cholesterol amount-1/5.times.triglyceride amount